The disclosure relates, in general, to the identification of transglutaminases and substrates therefore, and more particularly to the discovery and characterization of a microbial transglutaminase from Kutzneria albida. 
Elucidating the details of enzyme activity and specificity is important for understanding the physiological function of enzymes and for biotechnological applications of the reactions catalyzed by enzymes. For example, transglutaminases belong to a large family of related enzymes, including microbial and mammalian transglutaminases. Transglutaminases catalyze cross-linking between two polypeptide or peptide chains by forming an isopeptide bond between a gamma-carboxamide group of a glutamine residue and an epsilon-amino group of a lysine residue. Elucidating the details of transglutaminase activity and specificity is important for biotechnological applications of the cross-linking reaction catalyzed by transglutaminases, for example, for modification of proteins for labeling, tagging, multi-protein complex formation, and the like.
To date, microbial transglutaminase is the most studied transglutaminase enzyme because of its small size, robust performance, stability, and the calcium independence of its activity. Several studies have shown that a broad variety of long alkylamines can substitute for the lysine substrate of transglutaminases and the simple dipeptide glutamine-glycine can serve as the glutamine substrate. These discoveries of lysine and glutamine substrates of transglutaminases have helped to develop a variety of tests for transglutaminase activity and practical assays for modification of proteins using transglutaminases. However, several challenges may still arise in the identification and characterization of known and novel transglutaminases. One challenge is the specificity of a particular transglutaminase for isopeptide bond formation may be too broad or too narrow for a particular application. Another challenge is transglutaminases having the same or similar substrate specificity may not be useful for orthogonal labeling strategies, or the like. Yet another challenge is the identification of substrates for uncharacterized or poorly characterized transglutaminases. Still other challenges may arise depending on factors associated with a given transglutaminase, such as the origin, specificity, activity, stability, the like, and combinations thereof.